Thin film resonator (TFR) technology makes possible thin film acoustic devices operable in the microwave range that are compatible with active semiconductor circuitry. They are compatible with such circuitry both in the sense of being similarly sized, and in that the thin film acoustic devices can be integrated with semiconductor devices on a common substrate.
A basic thin film resonator is composed of the three basic elements common to all piezoelectric resonators: a piezoelectric dielectric, electrodes for applying electric fields, and two reflecting surfaces for establishing a standing wave. In TFR technology, thin film metallic electrodes, typically composed of aluminum, serve both as electrodes and as reflecting surfaces with the metal/air interface at the top of the resonator serving as the primary source of reflection. Sandwiched between the two electrodes is a piezoelectric layer. As the bottom metallic layer is typically grown directly on a substrate, the acoustic cavity for the resonator is defined by the aluminum-substrate composite membrane. To approximate a conventional free plate resonator, and in order to provide for high frequency operation, that membrane should be of low mass. Accordingly, it is preferable that the substrate material underlying the membrane portion of the thin film resonator be removed. Moreover, coupling between the thin film resonator and the bulk silicon would negatively impact the Q characteristics of the resonator, thus also requiring removal of the silicon substrate beneath the resonator.
A variety of techniques have been employed for the purpose of removing the substrate material beneath the resonator. In one such technique, a silicon substrate is employed with a boron doped p+ layer implanted at the upper surface which is to be adjacent the bottom aluminum electrode. This p+ layer serves as an etch stop for a selective backside chemical etch, which is performed after the resonator is fabricated on the substrate. The p+ layer itself may also optionally be removed. In either case, a via in the substrate is formed beneath the resonator, leaving it unsupported except at its edges. In an alternative technique, the thin film resonator is formed on a semiconductor substrate, and then an under cutting etch is used through a hole in the piezoelectric layer to remove the substrate material beneath the resonator. In a further alternative, a cross-over air bridge metalization pattern is used to create an air gap between the silicon and the bottom electrode of the resonator.
All of these techniques for removing material beneath the resonator share the common disadvantage of requiring multiple additional process steps after formation of the resonator to etch or otherwise remove the material underneath the resonator. Furthermore, the largely unsupported resulting structures have low yields because of the additional and complex process steps. Because of the suspended nature of these structures they are subject to damage both during processing and subsequent thereto. As a result of this fragility, thinner piezoelectric, higher frequency devices become problematic. The need for removal of this substrate material also affects how closely such resonators may be spaced, since the remainder of the silicon substrate must maintain its structural stability.
Other applications of TFR technology are similarly limited. A further example is a stacked crystal filter which utilizes a five layer structure of three alternating metal electrodes with two piezoelectric layers sandwiched therebetween. To operate properly, this device also requires the silicon substrate beneath to be removed.
Other devices utilizing TFR technology do not require etching of the silicon beneath the devices, particularly when the resonators are to be acoustically coupled through the substrate. One such example can be found in the inventor's published PCT application, bearing International Publication Number WO93/24971 published Dec. 9, 1993. That application discloses an acoustically coupled antenna using an overmoded configuration. In that case, a thin film resonator is grown on either side of a silicon substrate. Each thin film resonator comprises a piezoelectric layer sandwiched between two metal, typically aluminum, electrodes. The silicon, or other semiconductor, substrate serves as a delay element between the two thin film resonators. When the silicon has a thickness substantially equal to a multiple of one-half wave length of the desired frequency of the predetermined frequency band, the two thin film resonators become acoustically coupled in that pre-determined frequency band. Thus, while the requirement for etching of the silicon substrate has been eliminated in this device in order to provide coupling between the resonators, it is useful primarily in this antenna configuration. This is due to the fact, as mentioned previously, that the presence of the silicon substrate under a single, uncoupled resonator prevents conventional free plate resonance, as well as affecting the resonator Q because of transmission of the acoustic radiation into the substrate. Further, the overmoded structure also places a constraint on the thickness on the silicon, since it must be an integral multiple of one-half the wave length of a desired frequency in the predetermined frequency band of the resonators.